Andy Cowles has told Gately's bandmates from the popular Irish boy band that he blames himself for leaving the singer alone in their holiday apartment on the island of Majorca.
A post-mortem showed Gately, 33, an Irishman, died as a result of a pulmonary edema, or fluid in the lungs, the Balearic Islands Superior Justice Tribunal said in a statement.
The pair had been out partying on Friday night in the island's capital of Palma and returned to their apartment in the upmarket resort of Port d'Andratx with Bulgarian man Georgi Petrov Dochev in the early hours of Saturday morning.
Britain's Daily Mirror newspaper reported that Cowles had gone to bed while Gately fell asleep on the living room sofa while watching television.
The paper said Cowles told Boyzone band members Ronan Keating and Keith Duffy, who flew to Majorca on Sunday after learning of Gately's death: "I will never forgive myself for this."
"He died and I couldn't help him. I feel like my life is over."
A close friend told the newspaper Gately's bandmates told Cowles not to blame himself as he sobbed over his partner's death.
"The guy is in a deep state of shock and he hasn't slept for a minute since this tragedy unfolded," the friend said.
"He is a total mess and everyone is worried about him.
"They told him that there is nothing he could have done and that he has to stop blaming himself."
His family hope his body will be able to be returned home to Dublin for a funeral next week.
Media reports have suggested that Gately and Cowles, who married in a civil union in 2006, had been out on an eight-hour drinking binge hours before the singer died.
However Gately's family believe that neither drugs nor alcohol were to blame.
"This sort of thing does happen, we are absolutely happy that there is no foul play," the family's lawyer Gerald Kean told GMTV.
Police in Majorca have said there were no signs of suspicious circumstances.
Cowles has been interviewed by police and insisted neither he nor Gately had been drunk.
"We have had wilder nights out in Dublin, and this was nothing out of the ordinary," he said.
"I just don't understand it. What hurts most is that he was only yards from me in the room next door and I didn't even know he was slipping away.
"If only I had gone in to check him."
Meanwhile, the Bulgarian who was at the singer's flat has spoken of his shock at finding Gately's "cold and pale" body slumped in a mysterious prayer-like position.
Dochev had slept at the apartment and found Gately in the living room after emerging from a bedroom around 1pm on Saturday.
"I am devastated," he told The Sun newspaper.
"I came out of the bedroom and found Stephen dead. I woke the husband. It is awful -- I am in shock and I'm ill with it. I haven't slept a wink.
"I was with the husband all yesterday. He was too upset to call the ambulance. He was holding Stephen, so I called them.
"We have talked to the police but I cannot talk about other things."
